1. Field of the Invention
The present invention relates to aqueous resin compositions excellent as a coating material applied on internal surfaces of cans. The present invention also relates to coating materials containing the aqueous resin composition, and coatings provided thereby. Furthermore the present invention relates to metal plates coated with the coating material.
Furthermore the present invention relates to aqueous resin compositions containing polyester resin and phenol resin to be excellent in settability, flexibility, retort resistance, and extractability (hygienicity), particularly suitable for a can coating material, and to metal plates coated with the aqueous resin composition. The polyester resin has the following structural formula:

wherein R: H, alkyl or an aryl group,                X: an organic segment having at least one hydroxyl group or carboxyl group (including a carboxylic anhydride), and        n: 1 to 3and the polyester resin is capable of dispersion in water when a carboxyl group in a molecular chain thereof is neutralized with a basic compound.        
2. Description of Related Art
Coating materials applied on internal surfaces of food and beverage cans are required not to be toxic or provide pollutant effluents when they are dumped or recycled, and they are also required to endure processings of cans being produced, vapor generated in retort processes, heat, salt in contents, and acid.
Conventionally, coating materials mainly provided by epoxy resin have often been used to coat internal surfaces of cans. In particular, a disperse-in-water-type (or aqueous) epoxy-acrylic coating material has increasingly substituted for solution-based coating materials and used as a material for coating internal surfaces of cans to protect natural environments and improve working environments. Epoxy-acrylic resin can maintain uniform dispersibility in water and the resin that is applied on a can's internal surface provides excellent flexibility, excellent retort resistance, and the like. It has conventionally also been regarded as excellent in hygienicity for human body.
The epoxy-acrylic coating material, however, contains bisphenol-A as a source material, which has been pointed out as a possible extrinsic-factor endocrine disrupter, and in recent years in the field of foods in particular there is an increased demand to avoid using bisphenol-A for coating materials applied on internal surfaces of cans. Thus there is a demand for developing an aqueous coating material substituting for bisphenol-A. However, there is still not obtained a sufficiently suitable composition for the coating material.
For example, as described in Japanese Patent Laying-Open Nos. 9-296100, 11-61035, 11-124542, 11-236529, and 2000-26709, it is desirable that dicarboxylic acid, glycol, polycarboxylic monoanhydride or the like be used to depolymerize polyester resin or open a ring of the former and add it to the latter to obtain a terminal of the resin with carboxyl groups. Furthermore, it has also been proposed that the medium product is dispersed in water by neutralizing a carboxyl group in a molecule of the medium product to provide an aqueous coating resin composition containing amino resin and protective colloid.
Although these coating materials are excellent in settability and flexibility, amino resin is less hygienic and melamine formaldehyde resin in particular has poor retort resistance. It is possible to improve the poor retort resistance by using hydrophobic amino resin such as benzoguanamine-formaldehyde resin. However, the polyester resins obtained by the methods disclosed in the above publications have a carboxyl group concentrated at a terminal of a molecule thereof, resulting in poor disposition stability. Accordingly, protective colloid needs to be used to provide the coating material with stability. The protective colioid, however, impairs retort resistance and hygienicity. Furthermore, with its poor disposition stability, the coating material that is sprayed to coat a can's internal surface is disadvantageously stuck in the spray's nozzle or drips after it is applied.
Japanese Patent Laying-Open No. 11-315251 proposes combining polyester resin and resol-type phenol resin to provide a coating material improved in settability, flexibility, retort resistance, and hygienicity. However, the resol-type phenol resin described in the publication is, although partially, alkyl-etherified and thus exhibits poor dispersibility in water. To disperse the resin in water and maintain its dispersibility, protective colloid, a surfactant or the like is still required, which results in reduced retort resistance. The phenol resin disclosed in the publication can be used to achieve a higher level of settability with polyester resin than conventional phenol resin, although the higher settability is still lower than settability of a coating material formed of a combination of amino resin and polyester resin or that of phenol resin and epoxy resin and it can thus result in insufficient retort resistance. The publication also does not disclose any method or example specifically describing using these as an aqueous coating material.